1. Field of the Invention
The present invention relates to systems and methods for controlling spacecraft, and in particular to a system and method for operating a momentum bias controlled spacecraft in an inverted orientation without requiring any control law, or momentum wheel modifications.
2. Description of the Related Art
Various systems of attitude steering and momentum management for spacecraft are known today. These spacecraft use different systems of momentum wheel arrays for various orbit geometries and mission purposes.
Momentum bias stabilized spacecraft are very desirable due both to the inherent attitude stability afforded by the gyroscopic stiffness associated with the momentum and to the yaw attitude sensing facilitated by orbital coupling with the momentum bias. The orbital coupling with the momentum bias allows yaw attitude (which is difficult to measure with sensors) to be deduced from a roll attitude sensor.
Typically, the momentum bias mechanization is only useful for small attitude motion control about a nominal attitude with the bias nominally orbit normal. However, for selected spacecraft applications, it can be often useful to operate the spacecraft in an upright orientation (pitch axis anti-orbit-normal) at some times during the mission, and in an inverted orientation (pitch axis orbit-normal) during other portions of the mission. For example, this capability is useful in establishing new antenna coverage areas or in providing for other new missions.
Further, there are a large number of momentum bias stabilized spacecraft presently on orbit. As a result of proliferation in the number and uses of spacecraft, coupled with the aging and replacement of such satellites, the desire to move them and invert the attitude to establish new antenna coverage areas is becoming more frequent.
Early momentum bias spacecraft were controlled by thrusters and magnetic torquers that apply external torques. The attitude control of such spacecraft is unstable when inverted because the relative sign of the momentum bias and orbit rate rotation is reversed. Consequently, to stabilize such a spacecraft in the inverted state it is necessary to (1) reverse the direction of the momentum wheel rotation, or (2) design and implement a new control compensation, or (3) both (1) and (2).